Lubricants, such as lubricating oils and greases, are subject to deterioration at elevated temperatures, extreme contact pressures, or upon prolonged exposure to the elements. Such deterioration is evidenced in many instances by an increase in acidity and viscosity. It can cause metal parts to corrode and often leads to a loss of lubrication properties resulting in wear at the surfaces being lubricated, e.g. metal engine parts and the like.
A variety of additives have been developed to provide, antioxidant, antiwear, and deposit control properties etc, to these lubricants. Additives have also been developed to modify the lubricity and load bearing properties of the lubricant. For example, zinc dialkyldithiophosphates (ZDDP) have been used as antifatigue, antiwear, antioxidant, extreme pressure and friction modifying additives for lubricating oils for many years. However, ZDDPs are subject to several drawbacks due to the presence of zinc and phosphorus. For example, the presence of zinc contributes to emission of particulates in the exhaust.
Reducing friction between moving parts is of course a fundamental role of lubricants. This is especially significant in internal combustion engines and power transmission systems found in cars and trucks, for example, in part because a substantial amount of the theoretical mileage lost from a gallon of fuel is traceable directly to friction.
A variety of friction modifiers are widely known and used, including for example, fatty acid esters and amides, and organo molybdenum compounds, such as molybdenum dialkyldithiocarbamates, molybdenum dialkyl dithiophosphates, molybdenum disulfide, tri-molybdenum cluster dialkydithiocarbamates, non-sulfur molybdenum compounds and the like. Molybdenum friction modifiers are widely known and are effective over a broad temperature range, especially upon reaching temperatures of 120° C. or higher where chemical transformations form Mo-Sulfide glass coatings on surfaces. Molybdenum compounds however have some drawbacks, for example they can complex and interfere with dispersants and like other metal containing compounds, may suffer from particulate formation etc, as seen, for example, with the zinc anti-wear additive above. It is therefore desirable to reduce the amount of such friction modifiers in lubricants.
U.S. Pat. No. 5,333,470 discloses alkylated citric acid adducts, i.e., citrate esters, as antiwear and friction modifying additives for fuel and lubricants formed by reacting citric acid with 1, 2 or 3 equivalents of an alcohol. The anti-wear properties and friction reduction of compound mixtures derived from citric acid and oleyl alcohol are demonstrated.
U.S. Pat. No. 7,696,136 discloses lubricant compositions containing esters of hydroxy carboxylic acids, such as citrates and tartrates, which are useful as non-phosphorus-containing, anti-fatigue, anti-wear, extreme pressure additives for fuels and lubricating oils. The esters are used alone or in combination with a zinc dihydrocarbyldithiophosphate or an ashless phosphorus-containing additive, such as trilauryl phosphate or triphenylphosphorothionate. The addition of short chain esters, such as tri-ethyl citrate, borated tri-ethyl citrate and di butyl tartrate are shown to allow one to reduce the amount of ZDDP while maintaining good anti-wear properties.
It has now been found that while certain short chain esters of U.S. Pat. No. 7,696,136, e.g., tributyl citrate, can provide a modest decrease in friction coefficient of a lubricating oil, e.g., when added to a lubricant base stock or a commercial lubricant oil such as commercially available SAE 10-40, SAE 10-20, SAE 5-30 automotive oils etc, a much greater effect is seen when the citrate is combined with certain metal based friction modifiers, such as molybdenum friction modifiers. The surprisingly large synergy seen allows one to significantly reduce the amount of metal containing additives in lubricants, such as lubricants used in engines and power transmission systems.